Dyes having chromophores containing multiple non-aromatic conjugated double bonds have been important as infrared absorbing dyes. Infrared absorbing dyes have many utilities, depending on the nature of the specific dye. These include sensitization of silver halide emulsions and photopolymers, security marking, and laser imaging.
Developing methods for extending the length of linear conjugated systems and thereby extending the wavelength response of major classes of dyes has been a long standing problem in the dye synthesis art. For example, Fischer's base (i.e., 1,3,3-trimethyl-2-methyleneindoline) is the starting material for many polymethine, cyanine, and merocyanine dyes. Recently a vinylog of Fischer's base, 1,3,3-trimethyl-2-(2-propenylidene)indoline was reported, but could not be isolated in pure form due to side reactions during its synthesis (Khan, M. N.; Fleury, J.-P.; Baumlin, P.; Hubschwerlen, C. Tetrahedron 1985, 41, 5341 and Hubschwerlen, C.; Fleury, J. P. Tetrahedron 1977, 33, 761).
U.S. Pat. No. 4,942,141 discloses infrared absorbing squarylium dyes. The formula claimed therein encompasses certain squarylium dyes related to but not included within the present invention. There is no specific teaching of the advantages achieved by providing rigidized rings within the structure according to the present invention.